January 21, 2014. It takes a village of diverse neurons to make a brain, and among the villagers, parvalbumin (PV)-containing interneurons hold a special position. Comprising 25 percent of interneurons in primate cortex, these cells are critical synchronizers of neural activity. Rodent studies find them to be involved in timing neural activity, plasticity, and sustaining newborn neurons, and in schizophrenia, postmortem brain studies find signs of weakened inhibition by PV interneurons. Here, SRF reviews recent developments in PV interneuron biology, including two studies specific to schizophrenia, which bolster the proposition that PV interneurons are structurally sound but functionally compromised in the disorder.

Dysfunction in a circuit nexus
“We have to think about these neurons at the level of the circuit in which they are embedded,” said David Lewis of the University of Pittsburgh, Pennsylvania, who was involved in both studies. Lewis has been a driving force in identifying molecular abnormalities in PV interneurons and their pyramidal neuron counterparts in postmortem studies of people with schizophrenia (see SRF interview).

In the cortex, PV interneurons inhibit multiple pyramidal cells, which then feed back onto PV interneurons with excitatory inputs. This circuit creates a cycle of excitation and inhibition that gives rise to γ oscillations—coordinated fluctuations in neural activity across the brain at about 40 Hz (see SRF related news report)—which appear to be important for cognition. PV interneurons provide a synchronizing signal to many pyramidal cells, and the duration of their inhibitory inputs sets the timing of the rhythmic activity.

Lewis and colleagues have uncovered molecular differences in postmortem brain samples from people with schizophrenia that suggest that, in dorsolateral prefrontal cortex (DLPFC), PV interneurons do not suppress their pyramidal cell targets as they should: For example, PV interneurons are deficient in GAD67, the enzyme that makes the inhibitory neurotransmitter γ-aminobutyric acid (GABA), and their pyramidal cell targets show a decrease in GABA receptors (Lewis et al., 2012). Furthermore, Beretta and colleagues have found fewer numbers of perineuronal nets surrounding PV interneurons, which may protect the cells from the ravages of oxidative stress that come with their rapid firing (see SRF related news report). These and other alterations may undermine PV interneuron function and contribute to the disrupted γ oscillations—and cognitive difficulties—observed in schizophrenia (Uhlhaas et al., 2012; see SRF related news report).

Not only do PV interneurons run low on GAD67, but also on PV itself. This makes identifying them tricky, because PV is the marker of choice for these interneurons. The new studies address this with a careful accounting of PV levels in specific synapses and with an altogether different marker of a potassium channel subunit that selectively labels PV interneurons.

More molecules
The first study, from the Lewis lab with first authors Jill Glausier and Ken Fish, localizes the PV deficit to the axon terminals where the GABA-releasing machinery resides. Published online November 12 in Molecular Psychiatry, the study applied a technique that precisely pinpoints the location of three different labels: one for PV, one for the GABA-making enzyme GAD65, which is only found in axon terminals, and one for the GABA receptor α1 subunit, which characterizes synapses of basket cells, a subtype of PV interneurons that innervates the cell bodies and dendrites of pyramidal cells. Overlap among all three labels, then, specifically marked basket cell inputs.

In schizophrenia DLPFC samples, the basket cell inputs were just as numerous, in terms of density, as those in controls, but with decreased levels of PV. In combination with the decreased GAD67 found in these terminals (Curley et al., 2011), the findings suggest that the function rather than the structure of these inputs is altered in schizophrenia, and that finding ways to modulate these connections may restore the circuit.

The second study, published online October 30 in the American Journal of Psychiatry reports that PV interneurons lack the potassium channel subunits encoded by KCNS3 in schizophrenia. Earlier work had fingered KCNS3 as a specific marker for PV interneurons (Georgiev et al., 2012). The new study evaluated KCNS3 transcript levels in schizophrenia and found them lacking by two distinct methods: in-situ hybridization in one group of postmortem samples, and laser microdissection of PV interneurons followed by microarray measurement of mRNA in a different group of samples. Led by Takanori Hashimoto of Kanazawa University in Japan, in collaboration with Lewis, the study reports that KCNS3 levels were lower by 23 percent with in-situ hybridization and 40 percent lower by microarray.

First author Danko Georgiev and colleagues propose that this deficit would disrupt synchronous firing in cortical circuits. KCNS3 subunits normally offset excitation and so limit the duration of excitatory inputs. This would allow for only simultaneous inputs to evoke a spike. With fewer KCNS3 subunits around, however, the excitatory inputs received by PV interneurons would be prolonged, increasing the chances for overlap, and spiking, with unsynchronized inputs.

“So this reduced KCNS3 could, in and of itself, provide a molecular basis for impaired γ oscillations,” Lewis said. “What we're uncertain of right now is—how does it fit together with less GAD67?” He suggests that people with schizophrenia could have just one of these deficits, or, alternatively, both deficits may be required to bring about a disease-related pathology.

PV for plasticity
These molecular alterations seem to stem from disruptions to brain development rather than being consequences of schizophrenia onset, according to a recent analysis by Lewis and colleagues (Hoftman et al., 2013). For example, PV levels found in the postmortem studies do not vary according to duration of illness, but rather fall short of a normal increase occurring during adolescence.

But PV levels may be quite malleable in adulthood, according to a study published December 12 in Nature. Led by Pico Caroni of the Friedrich Miescher Institute in Basel, Switzerland, the study reports that experience alters PV levels in hippocampal PV interneurons in mice, which then dictates the plasticity state of the circuit. First author Flavio Donato and colleagues report that mice spending time in an enriched environment, with lots of things to play with and explore, had more PV interneurons, specifically basket cells, classified as “low-PV expressers,” than mice raised in a standard cage and mice that had been fear conditioned with electric shocks in their cage. Conversely, the fear-conditioned mice had a greater percentage of PV interneurons rated as “high-PV-expressers.” Changes in the types of synapses made onto PV interneurons also varied according to PV levels: Low-PV interneurons received mostly inhibitory inputs, whereas in high-PV interneurons, excitatory inputs predominated. Directly manipulating the activity of PV interneurons with optogenetics also brought about these changes. This suggests that experience can shift the hippocampal circuits between low-PV or high-PV configurations.

Further experiments suggested that low-PV configurations promoted the process of learning, in which new associations are made, but remain labile to take new information into account, whereas high-PV configurations allowed the establishment of strong memories. Interestingly, stimulating vasoactive intestinal peptide (VIP)-containing inputs to the hippocampus induced a low-PV configuration; the gene encoding a receptor for VIP has been linked to schizophrenia (see SRF related news report). The researchers suggest that finding ways to shift between low- and high-PV configurations could promote cognition.

Whether this insight might apply to schizophrenia depends on how much the hippocampal circuit in mice resembles that in humans. “Sometimes the literature seems to act as if a PV neuron is a PV neuron is a PV neuron, but we have to pay attention to species differences,” Lewis said. In the mouse cortex, for example, 50 percent of interneurons express PV, compared to 25 percent in primates, including humans, and this could influence species-specific differences in the resulting circuits.

Still, two other mouse studies point to additional roles for PV interneurons worth noting. One, published November 20 in Nature, is in keeping with their role as synchronizers. Led by Cyril Herry at the University of Bordeaux, France, the researchers found that optogenetically inhibiting PV interneurons in mouse cortex resets activity in their target pyramidal cells, inducing them to fire simultaneously. First author Julien Courtin and colleagues found that this promoted theta oscillations, which vary more slowly than γ oscillations, and drove the expression of a fear memory.

Another paper, published November 10 in Nature Neuroscience, reports that PV interneurons support newly born neurons in the adult hippocampus of mice. Hongjun Song and Guo-li Ming of Johns Hopkins University in Baltimore, Maryland, joined forces with Nicolas Toni of the University of Lausanne, Switzerland, to study PV interneuron involvement in adult neurogenesis. They found that suppressing PV interneuron activity led to a die-off of newborn neurons, whereas increased activity promoted their survival. In contrast, the group’s earlier study found that PV interneurons suppress stem cells from making new neurons in the first place (Song et al., 2012). Activity in PV interneurons, then, could convey a circuit’s need for new neurons.

Therapeutic avenue
Finding ways to selectively modulate PV interneurons seems like a possible therapeutic strategy for schizophrenia. One recent idea comes from a study of the neuregulin-ErbB4 signaling pathway, which is found primarily in PV interneurons and has been linked to schizophrenia through genetics and animal studies (e.g., see SRF related news report). Led by Andres Buonanno of the National Institute of Child Health and Human Development in Bethesda, Maryland, and published online November 11 in Proceedings of the National Academy of Sciences, the study reports that neuregulin binding to the ErbB4 receptor on PV interneurons in the rat hippocampus can activate the internalization of GABA receptors in the hippocampus. First authors Robert Mitchell and Megan Janssen found that this action, however, did not involve the typical tyrosine kinase activity of ErbB4. This suggests a new mode of signaling that may fine-tune inhibitory inputs onto PV interneurons and affect the inhibitory network in the hippocampus. Though GABA receptor deficits have been noted mostly for pyramidal cells in the neocortex in schizophrenia, the results suggest that the GABA receptors in PV interneurons could matter for hippocampus function, which also appears to be compromised in schizophrenia (see SRF Live Discussion).—Michele Solis.

Our Conte Center is focused on the transcriptome, connectome, and plasticity of PV cells
as the neurodevelopmental basis for mental illness. Their
maturational state dictates the degree of plasticity in developmental critical periods, and
now we know, from Donato et al., in adult learning. Once plasticity is opened by PV cells' function, it closes
when they mature ("high PV" state), including the tightening of the perineuronal nets (PNNs) around them.

In schizophrenia, PV cells may remain in the "low PV" weak PNN state for some time longer than
normal, suggesting, interestingly, that developmental plasticity may be prolonged (i.e., neural circuits fail to stabilize when they normally should). PV cell
maturation may potentially be controlled by Otx2 secreted from the choroid plexus,
which would link enlarged ventricles to impaired PV cells in the brain in schizophrenia (see
Spatazza et al., 2013).